The increased use of telephone twisted pair wiring for data communications has resulted in a push for faster modems and improved signaling protocols compatible with the existing infrastructure of the Public Switched Telephone Network (PSTN). For example, the emerging variety of Digital Subscriber Line (DSL) communications protocols including Asymmetric Digital Subscriber Line (ADSL), Symmetric Digital Subscriber Line (SDSL), High Bit Rate Digital Subscriber Line (HDSL) and Very High Rate Digital Subscriber Line (VDSL) promise tremendous gains in throughput over twister pair wiring. Another example includes the wide spread deployment and use of the “56K” technology developed by U.S. Robotics and Rockwall/Lucent.
These and other developing technologies will continue to increase the speed of data and digital voice transmissions in the communications network. In a dial-up network, a modulator/demodulator or “modem” is often used to connect local and remote computers to each other over twisted pair wiring. In essence, a modem provides the communications device for transmitting data from one user system, such as a desktop, notebook or palmtop computer, to another user system at a remote end of a connection. Some modems have the capability to process voice for telephony applications.
The use of such protocols and the resulting increase in data transmission rates have created new challenges and concerns regarding the reliability and integrity of the data as it travels over the communications link. For example, it may be necessary to control the characteristics of the outgoing signals in order to assure a reliable end-to-end transmission. Such characteristics may include the amplitude and linearity of the outgoing signals as well as noise levels and frequency spectrum. Typically, a modem includes a Digital Signal Processor (DSP), an Analog Front End (AFE), a memory chip, interface blocks and power management circuitry among other components which are employed to accomplish such signal management functions.
Some DSPs contain the Analog-to-Digital (A/D) and Digital-to-Analog (D/A) circuitry used to convert analog signals to their digital counterpart and vice versa. In some instances the digital data is companded to increase the data rate and the useful bandwidth. In essence, the DSP provides the modulation and demodulation functions in the digital domain while the AFE is the interface mechanism between the user system and the PSTN.
Depending on line conditions, it may be necessary to control the amplitude and/or volume of outgoing signals for reliable communications using a given protocol. For example, mismatches in line impedance between the communications device, e.g. the modem, and the transmission line can result in nonlinearities and harmonic distortion. Such effects may also be amplified by the nonlinear components in the modem or a Plain Old Telephone System (POTS) coupled to the same wire line pair as the modem. Examples of such nonlinear components include Zener diodes, transistors, varistors, triacs and other devices used for over voltage protection, side-tone generation and over-volume protection, for example.
Power and volume control can be achieved by providing a certain amount of gain programmability in the AFE. For example, by sensing the line condition or determining the required volume setting, the DSP is able to generate appropriate control signals that control amplitudes at the AFE. In one application over twisted pair wiring, typical range of gain control may vary between +12 to −24 dB with gain resolution of +/−2 dB.
Various devices and methods for controlling the gain of outgoing signals have been used and implemented in the past. For example, FIG. 1 shows a prior art programmable gain amplifier 10 implemented as an operational amplifier 12 coupled to a resistor ladder R2. The programmable gain amplifier 10 operates in the analog domain to cause a signal at input terminal 22 to be amplified by the ratio of the resistor ladder R2 and the feedback resistance R1. Gain in the programmable gain amplifier 10 is achieved by selecting the gain settings through gain bits G0:G4 which, in turn, selects one of the resistance values RIN and RG1 through RG4, respectively.
An advantage of the programmable gain amplifier 10 is the ability to generate a relatively stable Signal-to-Noise Ratio (SNR) as the signal at output terminal 30 remains relatively flat for a desired gain setting. As the gain bits G0:G4 are selected, the ratio R1/R2 remains fixed so that the overall realized gain for signals at the input terminal 22 with respect to the output terminal 30 remains relatively constant.
Since the programmable gain amplifier 10 operates in the analog domain, it requires a large number of discrete components to implement. As such, the programmable gain amplifier 10 is a gain solution that consumes considerable board space within the communications equipment. For example, in one known solution, the approximate area consumed by the amplifier 10 for a gain range of between 0 to −24 dB in steps of 2 dB can approach 0.5 Ksqmils. The board space requirements increase with smaller step sizes as more resistance values are necessary to accommodate smaller variations in gain.
Thus, for applications where board space is limited, the gain programming amplifier 10 becomes impractical or impossible to implement. Examples of such applications include portable and palmtop computers systems among other small computing platforms. In addition, since the realized gain of the amplifier 10 is proportional to the ratio R1/R2, any mismatch in values between R1 and R2 means that different devices will have different gain factors at the output terminal 30. Such variances in devices are intolerable in certain applications where precise signal control is required.
While digital programmable gain amplifiers are available, they sacrifice the SNR advantages of analog implementations. What is needed is a way of controlling the power and volume of outgoing signals that fits the confines of limited footprints and board space restrictions found in many compact applications. A programmable gain solution that offers relatively stable SNR across the entire transmission bandwidth would provide numerous advantages.